A multistage vacuum pump generally comprises a pair of shafts each supporting plurality of rotor components. The shafts are located within a housing providing a stator for the pump. The housing comprises a gas inlet, a gas outlet and a to plurality of pumping chambers, with adjacent pumping chambers being separated by a partition member, generally in the form of a transverse wall. Fluid transfer channels connect the pumping chambers together.
Each pumping chamber houses a pair of Roots rotor components to provide a pumping stage of the pump. Each pair of rotor components is housed within a respective pumping chamber such that there is a small clearance between the rotor components and between each rotor component and an inner wall of the pumping chamber.
It is known, for example, from U.S. Pat. No. 6,572,351, EP 1,398,507 and US Patent Publication No. 2003/0133817, to form the housing of such a multistage vacuum pump from two half-shell stator components, which define the plurality of pumping chambers and the fluid transfer channels for conveying gas between the pumping chambers. In U.S. Pat. No. 6,572,351 and EP 1,398,507, the transfer channels are located within the partition members serving to separate adjacent pumping chambers, which has the effect of increasing the thickness of the partition members and thus undesirably increasing the overall length of the pump. In US 2003/0133817, the transfer channels extend circumferentially around the pumping chambers and partition members to connect adjacent pumping chambers together. However, this makes the transfer channels prone to blockage during manufacture, for example during a casting process.